---
title: SubInspector
description: "Sub Inspector is where a night’s haul gets judged. Point it at a folder, analyze everything, and separate keepers from rejects – by eye, by metrics, or both – without ever deleting a file. ### Loading & the Thumbnail Grid…"
url: "https://astro-pm.com/docs/sub-inspector-grading/"
updated: "2026-07-12"
category: "Astro PM App Guide / Analysis Tools & Calculators"
---

# SubInspector

Sub Inspector is where a night’s haul gets judged. Point it at a folder, analyze everything, and separate keepers from rejects – by eye, by metrics, or both – without ever deleting a file.

### Loading & the Thumbnail Grid

**Open Folder** (or drag a folder onto the panel) scans recursively for FITS/XISF subs. Each sub becomes a tile: an auto-stretched thumbnail with its filter and exposure, sub number, date, an accept/reject badge, and a two-row metrics strip (Score, SQM, Mean / HFR, HFD, FWHM, Stars, Eccentricity) that fills in once analysis runs. Two hover buttons on every tile: one opens the sub in the [Image Analysis panel](/docs/image-analysis-diagnostics/), the other opens a high-resolution preview window.

Display controls: **Size** (thumbnail height, 500-1000px) and **Stretch** (0.1x-4.0x histogram stretch – display only, never affects measurements).

[![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131157.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131157.png)

### Analyze: the Measurements

The **Analyze** button runs star detection over every loaded sub (cancellable, with progress) and computes per-image:

- **HFR / HFD** – half-flux radius/diameter, the workhorse focus metric.
- **FWHM** – profile width, tracks seeing and focus.
- **Star count** – collapses when clouds pass or focus runs away.
- **Eccentricity** – star elongation, 0 = round; rises with guiding error, wind, differential flexure.
- **Mean** – background level in ADU (gradients, passing high cloud, dawn).
- **Score** – a composite 0-100 quality score combining the metrics.
- **SQM** – sky quality, when your acquisition software wrote it into the headers.

### The Graph

**Show Graph** opens a time-series plot of any combination of the seven metrics across the whole session – checkboxes toggle each series. This is where trends jump out: focus drifting between autofocus runs, seeing degrading after midnight, the exact frame where clouds arrived. **Click a point** to select that sub in the grid; Shift+click selects a range.

[![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131417.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131417.png)

### Auto-Select: Threshold Grading

Next to the graph, each metric has a percentage slider (0-100%). The slider means “keep the best N% by this metric” – set HFR to 80% and the softest 20% of frames are flagged; set Stars to 90% and cloud-eaten frames drop. A sub is rejected if it fails *any* active threshold, and the colored **filter pills** above the grid show which rule caught how many. **Reset** returns every slider to 100% (keep all).

### Rejection Is Always Reversible

   [![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131454.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131454.png)

Rejecting – by slider, by clicking a tile, or Shift+click for a range – **moves files into a /rejected subfolder** inside the sub folder. Nothing is ever deleted. **View Rejected** flips the grid to show that folder’s contents, where **Unreject Selected** moves files back. Your stacking software simply never sees the rejects.

### Sessions, Sorting, and the Blink Player

   [![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131515-1024x683.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131515.png)

- **Sessions** – a hierarchical filter by observation night and mosaic panel (tri-state checkboxes, All/None shortcuts) so a multi-night folder can be reviewed night by night.
- **Sort** – by Name, Date, Filter, Score, SQM, Mean, HFR, FWHM, Stars, or Eccentricity, ascending or descending. Sorting by Score puts the worst frames together for fast triage.
- **Blink Player** – cycles frames rapidly like a classic blink comparator; drifting focus, wandering satellites, and passing cloud become instantly obvious.

   **Tip:** A fast nightly routine: Analyze → sort by Score ascending → eyeball the bottom of the pile → set Auto-Select sliders → done. A 300-sub night takes minutes.